In order to monitor the “charge state” of a battery during either a charging operation or when it is under load (discharging), it is necessary to determine the number of coulombs that are extracted from the battery or are input to the battery, i.e., the charging current and the discharge current. In order to do this, there must be some way to determine the current into or out of the battery. This is typically facilitated through the use of a sense resistor. This is a small value resistor that is inserted into the current path of the battery. By measuring the voltage across this sense resistor, the current there through can be determined, knowing the value of the resistor. Depending upon the accuracy required, this will determine the precision level required of the sense resistor. This resistor can be utilized for either high-side sensing or low-side sensing. In high-side sensing, the resistor is disposed between the positive or charging source and the battery and, in low-side sensing, the sense resistor is disposed between the negative terminal of the battery and ground.
Typically, the sense resistor is an external resistor to a charge monitoring integrated circuit. Thus, there will typically be required an additional two terminals on the integrated circuit package to accommodate this resistor. Further, this resistor must be large enough such that the voltage developed there across can be measured accurately and it must be able to dissipate the associated 12R losses, thus requiring the resistance value to be as small as possible. The accuracy of the sense resistor directly affects the accuracy of the charge estimation, and this requirement demands resistors with the lowest possible tolerance on the order of 0.1% or less. Ideally, the resistor accuracy must be better than the data converter resolution and, for a 15-bit converter, that is 0.003%. Typical resistor accuracy is on the order of 1% with 0.1% available at premium prices. This sense resistor must also be capable of conducting the full battery current without damage or significant heating. In battery safety circuits, the cost of an external sense resistor cannot be tolerated and the present method for sensing battery current is to use the RdsOn resistance of the FET devices that are fabricated on-chip. This method is extremely inaccurate because RdsOn varies widely from part to part and over temperature. The use of an on-chip resistor is also problematic due to the inaccuracy of the resistor and the large amount of power that must be dissipated on the surface of the chip.